Brake control device for vehicle

ABSTRACT

A brake control device is applied to a brake device that controls a front-wheel braking force and a rear-wheel braking force. The brake control device includes a ratio calculation circuit that calculates a target front and rear braking force distribution ratio based on a target pitch angle, and a brake control circuit that performs a stability control by operating the brake device based on the target front and rear braking force distribution ratio during braking.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 16/981,165 having a U.S. National Stage entry date of Sep. 15, 2020,which is the U.S. National Stage of PCT/JP2019/009471 filed Mar. 8,2019, and claims the benefit of Japanese Patent Application No.2018-066806 filed on Mar. 30, 2018, the disclosures of all of which arehereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to a brake control device for a vehiclethat controls a pitch angle of the vehicle during braking.

BACKGROUND ART

Patent Literature 1 describes an example of a device that changes afront and rear braking force distribution ratio to increase therear-wheel braking force when a traveling attitude of the vehiclebecomes a predetermined traveling attitude during braking. The front andrear braking force distribution ratio in this document is a ratio of therear-wheel braking force with respect to the front-wheel braking force,The rear-wheel braking force is the braking force applied to the rearwheels, and the front-wheel braking force is the braking force appliedto the front wheels.

CITATIONS LIST Patent Literature

-   Patent Literature 1: Japanese Unexamined Patent Publication No.    2017-109664

SUMMARY OF INVENTION Technical Problems

In the device described in Patent Literature 1, the front and rearbraking force distribution ratio is changed so that a vehicle is tiltedbackward (e.g., in a nose lift) if an attitude of the vehicle has apredetermined attitude during traveling. This means that the vehiclethat has been tilted forward (e.g., in a nose dive or in a nose down)due to deceleration begins to be tilted backward by changing the frontand rear braking force distribution ratio. In this case, a time lagoccurs from when change of the front and rear braking force distributionratio is inputted to when the ratio is actually changed and the vehiclebegins to be tilted backward. As a result, the pitch angle of thevehicle may fluctuate during braking.

Solutions to Problems

A brake control device for a vehicle for solving the above problems isapplied to a brake device configured to adjust a front-wheel brakingforce that is a braking force for front wheels of a vehicle, and arear-wheel braking force that is a braking force for rear wheels of thevehicle. The brake control device includes, a ratio calculation circuitthat calculates a target front and rear braking force distribution ratiothat is a target value of the front and rear braking force distributionratio based on the target pitch angle, the target value of a pitch angleof the vehicle during braking is a target pitch angle, and a ratio ofthe rear-wheel braking force with respect to the front-wheel brakingforce is a front and rear braking force distribution ratio, and a brakecontrol circuit that performs a stability control by operating the brakedevice based on the target front and rear braking force distributionratio during braking.

In the above configuration, the brake device is operated from whenbraking starts as the stability control. That is, from when brakingstarts, the front-wheel braking force and the rear-wheel braking forceare adjusted so that the actual front and rear braking forcedistribution ratio becomes the target front and rear braking forcedistribution ratio. The target front and rear braking force distributionratio is a ratio based on the target pitch angle. Therefore, comparedwith a case where the adjustment of the front-wheel braking force andthe rear-wheel braking force based on the target front and rear brakingforce distribution ratio is started after the pitch angle of the vehicledeviates from the target pitch angle, the fluctuation of the pitch angleof the vehicle during braking can be prevented.

The front and rear braking force distribution ratio is the ratio of therear-wheel braking force with respect to the front-wheel braking force.Therefore, an increase of the front and rear braking force distributionratio indicates a decrease of the distribution ratio of the brakingforce to the front wheels and an increase of the distribution ratio ofthe braking force to the rear wheels larger. On the contrary, a decreaseof the front and rear braking force distribution ratio indicates anincrease of the distribution ratio of the braking force to the frontwheels and a decrease of the distribution ratio of the braking force tothe rear wheels.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration view showing an outline of a vehicle includinga brake control device of an embodiment.

FIG. 2 is a schematic view showing a state in which a pitching moment isnot generated in the vehicle.

FIG. 3 is a schematic view showing a state in which a pitching moment isgenerated in the vehicle.

FIG. 4 is a block diagram showing a functional configuration of thebrake control device.

FIG. 5 is a graph showing a transition of an ideal front and rearbraking force distribution ratio.

FIG. 6 is a graph showing a relationship between the ideal front andrear braking force distribution ratio and a target front and rearbraking force distribution ratio.

FIG. 7 is a flowchart describing a processing routine executed to set acontrol front and rear braking force distribution ratio.

FIG. 8 is a flowchart describing a processing routine executed tocontrol the operation of the brake device.

FIG. 9 is an operation diagram when control is switched from a stabilitycontrol to a stable control.

FIGS. 10A to 10C are timing charts when control is switched from thestability control to the stable control.

FIG. 11 is an operation diagram when the target front and rear brakingforce distribution ratio used in the stability control is changed.

FIGS. 12A to 12C are timing charts when the target front and rearbraking force distribution ratio used in the stability control ischanged.

DESCRIPTION OF EMBODIMENT

Hereinafter, one embodiment of a brake control device for a vehicle willbe described with reference to FIGS. 1 to 12 .

FIG. 1 illustrates a vehicle equipped with a brake control device 50according to the present embodiment. The vehicle is provided with apower circuit 10 having a power source such as an engine and an electricmotor. The vehicle shown in FIG. 1 is a front-wheel drive vehicle.Therefore, the driving force output from the power circuit 10 istransmitted to the front wheels FL, FR of the wheels FL, FR, RL, RR. Thevehicle equipped with the brake control device 50 may be a rear-wheeldrive vehicle or a four-wheel drive vehicle.

The vehicle is provided with a plurality of braking mechanisms 11provided for the wheels FL, FR, RL, RR, and a brake device 15 thatcontrols the operation of each braking mechanism 11. Each brakingmechanism 11 includes a wheel cylinder 12 to which brake fluid issupplied, a rotating body 13 that rotates integrally with the wheels FL,FR, RL, RR, and a friction material 14 that relatively moves in adirection of moving toward and moving away from the rotating body 13.Then, in each braking mechanism 11, the force for pressing the frictionmaterial 14 against the rotating body 13 increases as the WC pressurePwc, which is the fluid pressure in the wheel cylinder 12, becomeshigher. Thus, braking force corresponding to the force for pressing thefriction material 14 against the rotating body 13 is applied to thewheels FL, FR, RL, RR. The braking force applied to the front wheels FL,FR by the operation of the braking mechanism 11 is referred to as the“front-wheel braking force BPf”, and the braking force applied to therear wheels RL, RR by the operation of the braking mechanism 11 isreferred to as the “rear-wheel braking force BPr.”

When a brake operation member 16 such as a brake pedal is operated bythe driver of the vehicle, the WC pressure Pwc in each wheel cylinder12, that is, the front-wheel braking force BPf and the rear-wheelbraking force BPr are increased so that the deceleration of the vehicleincreases as the braking operation amount X increases. In the presentembodiment, the WC pressure Pwc in each wheel cylinder 12, that is, thefront-wheel braking force BPf and the rear-wheel braking force BPr, isadjusted so that a pitch angle PA of the vehicle becomes a target pitchangle PATr by controlling the brake device 15 by the brake controldevice 50.

The vehicle including the brake control device 50 has an automatictraveling function. Even at the time of automatic braking duringautomatic traveling of the vehicle based on the automatic travelingfunction, the WC pressure Pwc in each wheel cylinder 12, that is, thefront-wheel braking force BPf and the rear-wheel braking force BPr areadjusted so that the pitch angle PA of the vehicle becomes the targetpitch angle PATr.

Next, pitching of the vehicle during braking will be described withreference to FIGS. 2 and 3 . FIG. 2 shows a vehicle 20 when alongitudinal acceleration of the vehicle 20 is “0” such as when thevehicle is stopped or when the vehicle is traveling at a constant speed.On the other hand, FIG. 3 shows the vehicle 20 during braking. Note thatin FIGS. 2 and 3 , the sprung load SWf on the front wheel side and thesprung load SWr on the rear wheel side are represented by white arrows.The sprung load is a load in a vertical direction input to thesuspension from the vehicle body by the vehicle weight and the pitchingmoment. Furthermore, of the sprung loads, the load input to springs 21 fand 21 r of the suspension, that is, the load that the springs 21 f and21 r bear is referred to as the “spring load”. The load in the verticaldirection on the road surface of the wheel is referred to as the “groundcontact load of the wheel”.

As shown in FIG. 2 , when the longitudinal acceleration Gx of thevehicle 20 is “0”, the length of the front wheel spring 21 f forming thesuspension for the front wheel is held at the length in which the sprungload SWf on the front wheel side and the reaction force of the frontwheel spring 21 f are balanced. Similarly, the length of the rear wheelspring 21 r forming the suspension for the rear wheel is held at thelength in which the sprung load SWr on the rear wheel side and thereaction force of the rear wheel spring 21 r are balanced. When neitherthe braking force nor the driving force is applied to the vehicle, thatis, when the vehicle is traveling by inertia, both the anti-dive forceand the anti-lift force are not generated in the vehicle. Furthermore,in such a case, no pitching moment is generated due toacceleration/deceleration of the vehicle. Therefore, the sprung loadsSWf and SWr from the vehicle body due to the vehicle weight become thespring loads.

When the vehicle is decelerated by the application of the braking forceto the vehicle 20, a pitching moment PM as indicated by a solid arrow inFIG. 3 is generated in the vehicle 20, and the vehicle 20 may tiltedforward. (e.g., in a nose dive.) The nose dive is a behavior of thevehicle in which the front portion of the vehicle 20 is displaceddownward and the rear portion of the vehicle 20 is displaced upward. Onthe other hand, the behavior of the vehicle in which the front portionof the vehicle 20 is displaced upward and the rear portion of thevehicle 20 is displaced downward is referred to as “nose lift”.

In the vehicle 20 including the brake control device 50, the geometriesof the suspension for the front wheel and the suspension for the rearwheel are set to satisfy the following two conditions. (Condition 1) Abraking force is applied to the front wheels FL, FR, and an anti-diveforce FAD that is a force in a direction of separating the front portionof the vehicle away from the front wheels FL, FR, that is, a directionof pushing up the front portion of the vehicle upward is generated.(Condition 2) A braking force is applied to the rear wheels RL, RR, andan anti-lift force FAL which is a force in a direction of approachingthe rear portion of the vehicle closer to the rear wheels RL, RR, thatis, a direction of pushing down the rear portion of the vehicle downwardis generated.

In FIG. 3 , the anti-dive force FAD and the anti-lift force FAL arerepresented by black arrows. The relationship between the anti-diveforce FAD and the front-wheel braking force BPf is determined by thespecifications of the vehicle, where the absolute value of the anti-diveforce FAD increases as the front-wheel braking force BPf increases.Furthermore, the relationship between the anti-lift force FAL and therear-wheel braking force BPr is determined by the specifications of thevehicle, where the anti-lift force FAL increases as the rear-wheelbraking force BPr increases

When both the anti-dive force FAD and the anti-lift force FAL aregenerated, the spring load on the front wheel side, which is the springload input to the front wheel spring 21 f, becomes the sum of the sprungload SWf on the front wheel side and the anti-dive force FAD.Furthermore, the spring load on the rear wheel side, which is the springload input to the rear wheel spring 21 r, is the sum of the sprung loadSWr on the rear wheel side and the anti-lift force FAL. The length ofthe front wheel spring 21 f during vehicle braking is a length in whichthe spring load on the front wheel side and the reaction force of thefront wheel spring 21 f are balanced. As shown in FIG. 3 , the directionof the anti-dive force FAD is opposite to the direction of the sprungload SWf on the front wheel side. Furthermore, the length of the rearwheel spring 21 r during vehicle braking is a length in which the springload on the rear wheel side and the reaction force of the rear wheelspring 21 r are balanced. As shown in FIG. 3 , the direction of theanti-lift force FAL is the same as the direction of the sprung load SWrof the rear wheel. When braking force is generated in both the frontwheels FL, FR and the rear wheels RL, RR, the vehicle decelerates, and apitching moment PM corresponding to the deceleration of the vehicle atthis time is generated in the vehicle. When braking forces of the samemagnitude are generated in the front wheels FL, FR and the rear wheelsRL, RR, in a typical vehicle, the anti-lift force FAL generated on therear wheels RL, RR side becomes larger than the anti-dive force FADgenerated on the front wheels FL, FR side. Alternatively, when brakingforces of the same magnitude are generated in the front wheels FL, FRand the rear wheels RL, RR, the reduction amount of the length of therear wheel spring 21 r by the anti-lift force FAL generated on the rearwheels RL, RR side becomes larger than the increase amount of the lengthof the front wheel spring 21 f by the anti-dive force FAD generated onthe front wheels FL, FR side. Therefore, even if the total sum of thebraking forces for each wheel FL, FR, RL, RR is the same, thecontraction amount of the rear wheel spring 21 r involved in vehiclebraking increases as the rear-wheel braking force BPr increases and theanti-lift force FAL increases, and consequently, the pitch angle PA tobackward of the vehicle tends to increase.

That is, assuming the ratio of the rear-wheel braking force BPr withrespect to the front-wheel braking force BPf is the front and rearbraking force distribution ratio DR (=BPr/BPf), the pitch angle PA ofthe vehicle is an angle corresponding to the front and rear brakingforce distribution ratio DR during braking. Specifically, as the frontand rear braking force distribution ratio DR decreases, the distributionof the braking force to the front wheels FL, FR increases, andtherefore, the ratio of the total force combining the anti-dive forceFAD and the anti-lift force FAL with respect to the total sum of thebraking forces applied to each of the wheels FL, FR, RL, RR decreases.As a result, the pitch angle PA of the vehicle indicates that thevehicle is in the nose dive. On the other hand, as the front and rearbraking force distribution ratio DR increases, the distribution of thebraking force to the rear wheels RL, RR increases, and therefore, theratio of the total force combining the anti-dive force FAD and theanti-lift force FAL with respect to the total sum of the braking forcesapplied to each of the wheels FL, FR, RL, RR increases. As a result, thepitch angle PA of the vehicle indicates that the vehicle is in the noselift.

Assume that a target value of the pitch angle during braking is a targetpitch angle PATr, and the front and rear braking force distributionratio DR corresponding to the target pitch angle PATr is a target frontand rear braking force distribution ratio DRTr. In this case, the pitchangle PA of the vehicle during braking may approach to the target pitchangle PATr by calculating the target front and rear braking forcedistribution ratio DRTr and controlling the front-wheel braking forceBPf and the rear-wheel braking force BPr based on this target front andrear braking force distribution ratio DRTr. The “front and rear brakingforce distribution ratio DR” referred to in the present embodiment isthe ratio of the sum of the rear-wheel braking force BPr applied to theleft rear wheel RL and the rear-wheel braking force BPr applied to theright rear wheel RR with respect to the sum of the front-wheel brakingforce BPf applied to the left front wheel FL and the front-wheel brakingforce BPf applied to the right front wheel FR.

Next, the brake control device 50 will be described with reference toFIGS. 1 and 4 .

As shown in FIG. 1 , the brake control device 50 receives signals fromvarious sensors such as a wheel speed sensor 101, a longitudinalacceleration sensor 102, and a stroke sensor 103, which are the same innumber as the wheels FL, FR, RL, and RR. The wheel speed sensor 101detects the wheel speed VW that is the rotation speed of thecorresponding wheel, and outputs a signal corresponding to the wheelspeed VW. The longitudinal acceleration sensor 102 detects alongitudinal acceleration Gx which is the acceleration in thelongitudinal direction of the vehicle, and outputs a signalcorresponding to the longitudinal acceleration Gx. The stroke sensor 103detects a braking operation amount X which is an operation amount of thebrake operation member 16, and outputs a signal corresponding to thebraking operation amount X. Then, the brake control device 50 controlsthe brake device 15 based on the signals input from the various sensors101 to 103.

The brake control device 50 calculates various parameters necessary forperforming the braking control. That is, the brake control device 50calculates the wheel deceleration DVW by time-differentiating the wheelspeed VW calculated based on the output signal of the wheel speed sensor101 and inverting the positive/negative sign of the result of the timedifferentiation. Furthermore, the brake control device 50 calculates thevehicle body speed VS of the vehicle based on the wheel speed VW of eachwheel FL, FR, RL, RR. Moreover, the brake control device 50 calculatesthe vehicle body deceleration DVS of the vehicle by time differentiatingthe calculated vehicle body speed VS and inverting the positive/negativesign of the result of the time differentiation. In addition, the brakecontrol device 50 calculates a gradient θ of the road surface on whichthe vehicle travels based on the longitudinal acceleration Gx and thevehicle body deceleration DVS.

FIG. 4 shows a functional configuration of the brake control device 50for controlling the operation of the brake device 15 during braking. Aload amount estimation circuit 51 of the brake control device 50calculates an estimated value LC of the vehicle load amount. Under thecondition that the driving force transmitted to the front wheels FL andFR, which are the driving wheels, is constant, the acceleration of thevehicle at the start of the vehicle is less likely to increase thelarger the vehicle weight. Therefore, for example, the load amountestimation circuit 51 calculates the estimated value WS of the vehicleweight based on the driving force transmitted to the front wheels FL andFR and the acceleration of the vehicle at the start of the vehicle.Then, assuming the vehicle weight in a state where there is no load isan initial vehicle weight WSB, the load amount estimation circuit 51calculates a value, which is obtained by subtracting the initial vehicleweight WSB from the calculated estimated value WS of the vehicle weight,as the estimated value LC of the vehicle load amount.

Aground contact load estimation circuit 52 of the brake control device50 calculates, based on the gradient θ of the road surface on which thevehicle travels and the estimated value LC of the vehicle load amountestimated by the load amount estimation circuit 51, the estimated valueof the ground contact load of the front wheels FL, FR as the groundcontact load FWf of the front wheels and calculates the estimated valueof the ground contact load of the rear wheels RL, RR as the groundcontact load FWr of the rear wheels.

That is, when the road surface is an uphill road, the weight componentsupported by the rear wheels RL, RR tends to increase and the weightcomponent supported by the front wheels FL, FR tends to decrease in thevehicle weight than when the road surface is not an uphill road. On theother hand, when the road surface is a downhill road, the weightcomponent supported by the rear wheels RL, RR tends to decrease and theweight component supported by the front wheels FL, FR tends to increasein the vehicle weight than when the road surface is not a downhill road.Therefore, when the road surface is an uphill road, the ground contactload estimation circuit 52 calculates the ground contact loads FWf, FWrso that the ground contact load FWf of the front wheels decrease and theground contact load FWr of the rear wheels increases, the larger theabsolute value of the gradient θ of the road surface. Furthermore, whenthe road surface is a downhill road, the ground contact load estimationcircuit 52 calculates the ground contact loads FWf, FWr so that theground contact load FWf of the front wheels increases and the groundcontact load FWr of the rear wheels decrease, the larger the absolutevalue of the gradient θ of the road surface.

In addition, the ground contact load of the wheel tends to easilyincrease the larger the vehicle load amount. In particular, it can beinferred that the larger the number of occupants in the front portion ofthe vehicle interior, the larger the load applied to the front wheelsFL, FR. Therefore, the ground contact load estimation circuit 52distributes the estimated value LC of the vehicle load amount to theload amount of the front portion of the vehicle and the load amount ofthe rear portion of the vehicle based on the number of occupants in thefront portion of the vehicle interior. For example, the ground contactload estimation circuit 52 distributes the estimated value LC of thevehicle load amount to the load amount of the front portion of thevehicle and the load amount of the rear portion of the vehicle so thatthe load amount of the front portion of the vehicle increases as thenumber of occupants in the front portion of the vehicle interiorincreases. Then, the ground contact load estimation circuit 52calculates the ground contact load FWf of the front wheels such that theground contact load FWf of the front wheels increases as the load amountof the front portion of the vehicle increases. Furthermore, the groundcontact load estimation circuit 52 calculates the ground contact loadFWr of the rear wheels such that the ground contact load FWr of the rearwheels increases as the load amount of the rear portion of the vehicleincreases.

A pre-braking pitch angle calculation circuit 53 of the brake controldevice 50 calculates a pre-braking pitch angle PAb that is the pitchangle PA of the vehicle at the time of non-braking of the vehicle. Thepre-braking pitch angle calculation circuit 53 calculates thepre-braking pitch angle PAb based on the ground contact load FWf of thefront wheels and the ground contact load FWr of the rear wheelscalculated by the ground contact load estimation circuit 52. In otherwords, immediately before the start of vehicle braking, when it can beconsidered that the driving force is not transmitted to the front wheelsFL, FR, which are the driving wheels, that is, when the vehicle isconsidered to be traveling by inertia, the length of the front wheelspring 21 f becomes shorter as the ground contact load FWf of the frontwheel increases and the length of the rear wheel spring 21 r becomesshorter as the ground contact load FWr of the rear wheel increases.Therefore, the pre-braking pitch angle calculation circuit 53 calculatesthe length of each spring 21 f, 21 r, based on the specifications of thevehicle such as the spring constant of the front wheel spring 21 f andthe spring constant of the rear wheel spring 21 r, so that the length ofthe front wheel spring 21 f becomes a value corresponding to the groundcontact load FWf of the front wheel and the length of the rear wheelspring 21 r becomes a value corresponding to the ground contact load FWrof the rear wheel. Then, the pre-braking pitch angle calculation circuit53 calculates the pre-braking pitch angle PAb based on the calculatedlengths of the springs 21 f and 21 r and the wheel base length of thevehicle. The pre-braking pitch angle PAb to forward increases as thelength of the front wheel spring 21 f shortens. That is, the pre-brakingpitch angle PAb to forward increases as the length of the rear wheelspring 21 r lengthens. That is, the pre-braking pitch angle PAb islikely to be a value in the nose dive direction the larger the groundcontact load FWf of the front wheels, that is, the smaller the groundcontact load FWr of the rear wheels. Furthermore, the pre-braking pitchangle PAb is likely to be a value in the nose lift direction the smallerthe ground contact load FWf of the front wheels, that is, the larger theground contact load FWr of the rear wheels.

A slip calculation circuit 54 of the brake control device 50 calculatesa slip value SLPf of the front wheels FL, FR and a slip value SLPr ofthe rear wheels RL, RR. The slip values SLPf and SLPr of the wheels arenumerical values of the degree of deceleration slip of the wheels, andincrease as the degree of deceleration slip of the wheels increases. Forexample, the slip calculation circuit 54 calculates a value obtained bysubtracting the vehicle body deceleration DVS of the vehicle from thewheel deceleration DVW of the wheel as the slip values SLPf, SLPr of thewheels.

An ideal distribution characteristic learning circuit 55 of the brakecontrol device 50 learns, based on the slip value SLPf of the frontwheels FL, FR and the slip value SLPr of the rear wheels RL, RR duringbraking, an ideal distribution characteristic that represents therelationship between the ideal front and rear braking force distributionratio DRI and the vehicle body deceleration DVS of the vehicle. Theideal front and rear braking force distribution ratio DRI is a front andrear braking force distribution ratio at which the front wheels FL, FRand the rear wheels RL, RR simultaneously lock during braking. FIG. 5 isa graph in which the horizontal axis represents the front-wheel brakingforce BPf and the vertical axis represents the rear-wheel braking forceBPr, and FIG. 5 shows an ideal braking force distribution ratio line LIwhich is a line representing the ideal distribution characteristic. Asshown in FIG. 5 , the ideal front and rear braking force distributionratio DRI changes so that the rear-wheel braking force BPr decreases asthe front-wheel braking force BPf increases. A specific learning methodof the ideal distribution characteristic of the vehicle will bedescribed later.

Returning to FIG. 4 , a target pitch angle setting circuit 56 of thebrake control device 50 sets the target pitch angle PATr during brakingimmediately before the start of vehicle braking. That is, in the case ofvehicle braking accompanying the driver's braking operation, the targetpitch angle setting circuit 56 sets the target pitch angle PATr whendetecting the start of operation of the brake operation member 16.Furthermore, in the case of automatic braking during automatic travelingof the vehicle, the target pitch angle setting circuit 56 sets thetarget pitch angle PATr when the brake control device 50 is requested tostart automatic braking.

The target pitch angle setting circuit 56 sets a target pitch anglereference value PATrB that serves as a reference of the target pitchangle PATr depending on whether it is vehicle braking accompanying thedriver's braking operation (manual braking) or is automatic brakingduring automatic traveling. The pitch angle PA when the vehicle isstationary in the non-loaded state is set as the reference pitch anglePAs.

The target pitch angle setting circuit 56 sets the target pitch anglereference value PATrB to forward during manual braking is more than thereference pitch angle PAs to forward. On the other hand, the targetpitch angle setting circuit 56 sets the target pitch angle referencevalue PATrB to forward during automatic braking is less than the targetpitch angle reference value PATrB to forward during manual braking.

Furthermore, the target pitch angle setting circuit 56 modifies thetarget pitch angle reference value PATrB based on the ideal distributioncharacteristic of the vehicle (i.e., ideal distribution characteristicgrasped by the brake control device 50) learnt by the ideal distributioncharacteristic learning circuit 55 and the gradient θ of the roadsurface on which the vehicle travels, and derives the modified value asthe target pitch angle PATr A method of modifying the target pitch anglereference value PATrB and deriving the target pitch angle PATr will bedescribed later.

When the target pitch angle PATr is set by the target pitch anglesetting circuit 56, a ratio calculation circuit 57 of the brake controldevice 50 calculates the target front and rear braking forcedistribution ratio DRTr That is, the ratio calculation circuit 57calculates the target front and rear braking force distribution ratioDRTr immediately before the start of vehicle braking. The ratiocalculation circuit 57 calculates the target front and rear brakingforce distribution ratio DRTr based on the pre-braking pitch angle PAbcalculated by the pre-braking pitch angle calculation circuit 53 and thetarget pitch angle PATr set by the target pitch angle setting circuit56.

The ratio calculation circuit 57 may calculate the target front and rearbraking force distribution ratio DRTr so that the distribution of thebraking forces to the front wheels FL, FR increases as the differencebetween the pre-braking pitch angle PAb and the target pitch angle PATrincreases. This occurs when the target pitch angle PATr to forward ismore than the pre-braking pitch angle PAb.

During braking, an amount of change in the pitch angle PA of the vehicleto forward is an amount corresponding to the sum of the anti-dive forceFAD which is a force corresponding to the front-wheel braking force BPfand the anti-lift force FAL which is a force corresponding to therear-wheel braking force BPr. Therefore, the ratio calculation circuit57 calculates the front and rear braking force distribution ratio DR asthe target front and rear braking force distribution ratio DRTr suchthat the sum of the anti-dive force FAD and the anti-lift force FALbecomes a value corresponding to the difference between the target pitchangle PATr and the pre-braking pitch angle PAb. That is, if the targetpitch angle PATr to forward is more than the pre-braking pitch angle PAbto forward, the ratio calculation circuit 57 calculates the target frontand rear braking force distribution ratio DRTr so that the pitch angleof the vehicle inferred from the anti-dive force FAD and the anti-liftforce FAL generated in the vehicle during braking becomes the targetpitch angle PATr.

Even if the target pitch angle PATr to backward is more than thepre-braking pitch angle PAb to backward, the difference between thepre-braking pitch angle PAb and the target pitch angle PATr may not beso large. When the difference is small as described above, the ratiocalculation circuit 57 calculates the target front and rear brakingforce distribution ratio DRTr so that the distribution of the brakingforce to the rear wheels RL and RR increases than when the difference islarge. The sum of the anti-dive force FAD and the anti-lift force FALcan be made different from the case where the difference is large byperforming the vehicle braking based on the target front and rearbraking force distribution ratio DRTr. As a result, the change of thepitch angle PA of the vehicle to backward may be prevented.

A ratio modification circuit 58 of the brake control device 50 includesa modification amount calculation circuit 581 that calculates amodification amount ΔDR of the front and rear braking force distributionratio DR based on the slip value SLPr of the rear wheels RL and RRcalculated by the slip calculation circuit 54, and an addition circuit582. The modification amount calculation circuit 581 sets themodification amount ΔDR to “0” when the slip value SLPTh of the rearwheels RL and RR is less than the determination slip value SLPTh. On theother hand, the modification amount calculation circuit 581 makes themodification amount ΔDR equal to a specified value smaller than “0” whenthe slip value SLPr of the rear wheels RL, RR is larger than or equal tothe determination slip value SLPth.

When the learning of the ideal distribution characteristic by the idealdistribution characteristic learning circuit 55 is not in time, theideal distribution characteristic grasped by the brake control device 50may deviate from the actual ideal distribution characteristic. In such acase, the lock tendency of the rear wheels RL, RR may be large eventhough the lock tendency of the front wheels FL, FR is not so large.When the lock tendency of the rear wheels RL, RR is large under asituation where the front-wheel braking force BPf and the rear-wheelbraking force BPr are adjusted with the target front and rear brakingforce distribution ratio DRTr calculated by the ratio calculationcircuit 57, the target front and rear braking force distribution ratioDRTr needs to be modified so that the rear-wheel braking force BPr isless likely to increase to suppress reduction of the stability of thevehicle behavior. Therefore, the determination slip value SLPTh is setso that whether or not the target front and rear braking forcedistribution ratio DRTr needs to be modified can be determined based onthe slip values SLPr of the rear wheels RL and RR.

The addition circuit 582 calculates the sum of the target front and rearbraking force distribution ratio DRTr calculated by the ratiocalculation circuit 57 and the modification amount ΔDR calculated by themodification amount calculation circuit 581 as the target front and rearbraking force distribution ratio DRTr after modification. Therefore,when the modification amount ΔDR is equal to the specified value, thetarget front and rear braking force distribution ratio DRTr aftermodification decrease than the target front and rear braking forcedistribution ratio DRTr before modification.

An ideal ratio calculation circuit 59 of the brake control device 50,based on the ideal distribution characteristic learned by the idealdistribution characteristic learning circuit 55, that is, the idealdistribution characteristic grasped by the brake control device 50,calculates the ideal front and rear braking force distribution ratio DRIat this time. That is, the ideal ratio calculation circuit 59, based onthe ideal braking force distribution ratio line LI shown in FIG. 5 ,calculates a ratio based on the vehicle body deceleration DVS of thevehicle at this time as the ideal front and rear braking forcedistribution ratio DRI.

A ratio determination circuit 60 of the brake control device 50determines a control front and rear braking force distribution ratio DRCbased on the target front and rear braking force distribution ratio DRTrcalculated by the ratio calculation circuit 57, the target front andrear braking force distribution ratio DRTr after modification calculatedby the ratio modification circuit 58, and the ideal front and rearbraking force distribution ratio DRI calculated by the ideal ratiocalculation circuit 59. The method for determining the control front andrear braking force distribution ratio DRC will be described later.

The brake control circuit 61 of the brake control device 50 controls theoperation of the brake device 15 based on the control front and rearbraking force distribution ratio DRC determined by the ratiodetermination circuit 60 during braking. That is, the brake controlcircuit 61 controls the brake device 15 so that the ratio of therear-wheel braking force BPr with respect to the front-wheel brakingforce BPf becomes the control front and rear braking force distributionratio DRC. A specific method of controlling the brake device 15 by thebrake control circuit 61 will be described later.

In the present embodiment, the control front and rear braking forcedistribution ratio DRC is the target front and rear braking forcedistribution ratio DRTr, and the control of the brake device 15 by thebrake control circuit 61 based on the control front and rear brakingforce distribution ratio DRC is referred to as the “stability control”.The control front and rear braking force distribution ratio DRC is theideal front and rear braking force distribution ratio DRI, and thecontrol of the brake device 15 by the brake control circuit 61 based onthe control front and rear braking force distribution ratio DRC isreferred to as the “stable control”.

Next, the learning process of the ideal distribution characteristic ofthe vehicle by the ideal distribution characteristic learning circuit 55will be described with reference to FIG. 5 .

The ideal distribution characteristic learning circuit 55 learns theideal distribution characteristic so that the deviation between the slipvalue SLPr of the rear wheels RL, RR and the slip value SLPf of thefront wheels FL, FR becomes small during braking. Specifically, when theslip value SLPr of the rear wheels RL, RR is larger than the slip valueSLPf of the front wheels FL, FR, the ideal distribution characteristiclearning circuit 55 determines that the lock tendency of the rear wheelsFL, FR is larger than the lock tendency of the front wheels FL, FR andthat there is a possibility that the rear wheels RL and RR may lockbefore the front wheels FL and FR. Therefore, the ideal distributioncharacteristic learning circuit 55 changes the ideal distributioncharacteristic so that the front-wheel braking force BPf increases. InFIG. 5 , assume that the ideal braking force distribution ratio line LIrepresenting the ideal distribution characteristic that the idealdistribution characteristic learning circuit 55 has grasped so far isrepresented by a broken line. In this case, when the ideal distributioncharacteristic learning circuit 55 determines that there is apossibility that the rear wheels RL, RR may lock before the front wheelsFL, FR, for example, the characteristic that can be represented by theideal braking force distribution ratio line LI indicated by a one-dotchain line in FIG. 5 is stored as the ideal distribution characteristic.

On the other hand, during braking, when the slip value SLPf of the frontwheels FL, FR is larger than the slip value SLPr of the rear wheels RL,RR, the ideal distribution characteristic learning circuit 55 determinesthat there is a possibility that the lock tendency of the front wheelsFL, FR is larger than the lock tendency of the rear wheels RL, RR andthat the front wheels FL and FR may lock before the rear wheels RL andRR. Therefore, the ideal distribution characteristic learning circuit 55changes the ideal distribution characteristic so that the rear-wheelbraking force BPr increases. In FIG. 5 , assume that the ideal brakingforce distribution ratio line LI representing the ideal distributioncharacteristic that the ideal distribution characteristic learningcircuit 55 has grasped so far is represented by a broken line. In thiscase, when the ideal distribution characteristic learning circuit 55determines that there is a possibility that the front wheels FL, FR maylock before the rear wheels RL, RR, for example, the characteristic thatcan be represented by the ideal braking force distribution ratio line LIindicated by a solid line in FIG. 5 is stored as the ideal distributioncharacteristic.

Next, a process executed when the target pitch angle setting circuit 56derives the target pitch angle PATr will be described.

The target pitch angle setting circuit 56 calculates the pitch anglemodification amount ΔPA corresponding to the ideal distributioncharacteristic of the vehicle learnt by the ideal distributioncharacteristic learning circuit 55 and the gradient θ of the roadsurface on which the vehicle travels. Then, the target pitch anglesetting circuit 56 calculates the sum of the target pitch anglereference value PATrB and the pitch angle modification amount ΔPA as thetarget pitch angle PATr.

That is, the target pitch angle setting circuit 56 derives the idealfront and rear braking force distribution ratio DRI at the initial stageof braking based on the ideal distribution characteristic, andcalculates the pitch angle modification amount ΔPA so that the pitchangle modification amount ΔPA increases the larger the derived idealfront and rear braking force distribution ratio DRI. Furthermore, thetarget pitch angle setting circuit 56 modifies the pitch anglemodification amount ΔPA calculated based on the ideal front and rearbraking force distribution ratio DRI at the initial stage of brakingaccording to the gradient θ of the road surface on which the vehicletravels. It is more difficult to change the pitch angle PA to forward ofthe vehicle when the road surface is an uphill road when the roadsurface is an uphill road than when the road surface is not an uphillroad. On the other hand, when the road surface is a downhill road, it iseasier to change the pitch angle PA to forward of the vehicle than whenthe road surface is not a downhill road. Therefore, the target pitchangle setting circuit 56 performs a modification to increase the pitchangle modification amount ΔPA when the road surface is an uphill road,and performs a modification to decrease the pitch angle modificationamount ΔPA when the road surface is a downhill road.

Next, with reference to FIGS. 6 and 7 , a processing routine executedwhen the ratio determination circuit 60 determines the control front andrear braking force distribution ratio DRC will be described. Thisprocessing routine is executed every predetermined control cycle duringbraking.

As shown in FIG. 7 , in this processing routine, the ratio determinationcircuit 60 determines whether the vehicle body deceleration DVS of thevehicle is less than or equal to a switching deceleration DVSTh (S11).The switching deceleration DVSTh is a determination value fordetermining the timing at which control is switched from the stabilitycontrol to the stable control.

Here, the switching deceleration DVSTh will be described with referenceto FIG. 6 . In FIG. 6 , the broken line is the ideal braking forcedistribution ratio line LI, and the solid line is the target brakingforce distribution ratio line LTr which is a line representing therelationship between the target front and rear braking forcedistribution ratio DRTr and the vehicle body deceleration DVS. In FIG. 6, a plurality of constant deceleration lines A1, A2, A3 are shown by aone-dot chain line. The first constant deceleration line A1 is a lineconnecting points indicating the front-wheel braking force BPf and therear-wheel braking force BPr when the vehicle body deceleration DVSbecomes the first deceleration DVS1. The second constant decelerationline A2 is a line connecting points indicating the front-wheel brakingforce BPf and the rear-wheel braking force BPr when the vehicle bodydeceleration DVS becomes the second deceleration DVS2. Furthermore, thethird constant deceleration line A3 is a line connecting pointsindicating the front-wheel braking force BPf and the rear-wheel brakingforce BPr when the vehicle body deceleration DVS becomes the thirddeceleration DVS3. The first deceleration DVS1 is larger than the secondand third decelerations DVS2 and DVS3, and the third deceleration DVS3is smaller than the first and second decelerations DVS1 and DVS2.

In the graph shown in FIG. 6 , the second constant deceleration line A2passes through a point where the ideal braking force distribution ratioline LI and the target braking force distribution ratio line LTrintersect with each other. That is, in a case where the stabilitycontrol using the target front and rear braking force distribution ratioDRTr is performed, when the vehicle body deceleration DVS of the vehicleexceeds the second deceleration DVS2, the lock tendency of the rearwheels RL, RR increases than the lock tendency of the front wheels FL,FR, and the stability of the vehicle behavior may reduce. Therefore, theswitching deceleration DVSTh is set to a value corresponding to thevehicle body deceleration (the second deceleration DVS in the exampleshown in FIG. 6 ) when the ideal braking force distribution ratio lineLI and the target braking force distribution ratio line LTr intersect.In the present embodiment, the switching deceleration DVSTh is set to avalue equal to the vehicle body deceleration when the ideal brakingforce distribution ratio line LI and the target braking forcedistribution ratio line LTr intersect.

The switching deceleration DVSTh is set based on the ideal braking forcedistribution ratio line LI representing the ideal distributioncharacteristic grasped by the brake control device 50 and the targetbraking force distribution ratio line LTr. When the ideal distributioncharacteristic learning circuit 55 learns the ideal distributioncharacteristic, the vehicle body deceleration when the ideal brakingforce distribution ratio line L1 representing the ideal distributioncharacteristic grasped by the brake control device 50 and the targetbraking force distribution ratio line LTr intersect changes. Therefore,when the ideal distribution characteristic grasped by the brake controldevice 50 changes, the switching deceleration DVSTh changes. Even if theideal distribution characteristic grasped by the brake control device 50does not change, the switching deceleration DVSTh changes when thetarget front and rear braking force distribution ratio DRTr calculatedby the ratio calculation circuit 57 changes. The setting of theswitching deceleration DVSTh is also performed by the ratiodetermination circuit 60.

Returning to FIG. 7 , when the vehicle body deceleration DVS is lessthan or equal to the switching deceleration DVSTh (S11: YES), the ratiodetermination circuit 60 determines whether the slip value SLPr of therear wheels RL and RR calculated by the slip calculation circuit 54 islarger than the determination slip value SLPTh (S12). For example, instep S12, determination may be made that the slip value SLPr is largerthan the determination slip value SLPTh when the average value of theslip value SLPr of the left rear wheel RL and the slip value SLPr of theright rear wheel RR is larger than the determination slip value SLPTh.Furthermore, in step S12, determination may be made that the slip valueSLPr is larger than the determination slip value SLPTh when at least oneof the slip value SLPr of the left rear wheel RL and the slip value SLProf the right rear wheel RR is larger than the determination slip valueSLPTh. When the slip value SLPr is larger than the determination slipvalue SLPTh, the ideal distribution characteristic learning circuit 55has not learnt the ideal distribution characteristic in time, anddetermination is made that increase of the rear-wheel braking force BPrneeds to be limited by making the front and rear braking forcedistribution ratio DR smaller. Therefore, when the slip value SLPr islarger than the determination slip value SLPTh (S12: YES), the ratiodetermination circuit 60 selects the target front and rear braking forcedistribution ratio DRTr after modification calculated by the ratiomodification circuit 58 as the control front and rear braking forcedistribution ratio DRC (S13). This makes it possible to make the controlfront and rear braking force distribution ratio DRC smaller as comparedwith a case where the target front and rear braking force distributionratio DRTr before modification calculated by the ratio calculationcircuit 57 is selected as the control front and rear braking forcedistribution ratio DRC. Thereafter, the ratio determination circuit 60temporarily ends the present processing routine.

On the other hand, when the slip value SLPr is less than or equal to thedetermination slip value SLPTh (S12: NO), the ratio determinationcircuit 60 selects the target front and rear braking force distributionratio DRTr before modification calculated by the ratio calculationcircuit 57 as the control front and rear braking force distributionratio DRC (S14). Thereafter, the ratio determination circuit 60temporarily ends the present processing routine.

On the other hand, when the vehicle body deceleration DVS is larger thanthe switching deceleration DVSTh in step S11 (NO), the ratiodetermination circuit 60 determines whether the target front and rearbraking force distribution ratio DRTr after modification is selected asthe control front and rear braking force distribution ratio DRC (S15).When the target front and rear braking force distribution ratio DRTrafter modification is selected, determination is made that the controlfor limiting the increase of the rear-wheel braking force BPr hasalready been executed to suppress the reduction of the stability of thevehicle behavior.

Therefore, when the target front and rear braking force distributionratio DRTr after modification is selected (S15: YES), the ratiodetermination circuit 60 once ends the present processing routine. Onthe other hand, when the target front and rear braking forcedistribution ratio DRTr after modification is not selected (S15: NO),the ratio determination circuit 60 selects the ideal front and rearbraking force distribution ratio DRI as the control front and rearbraking force distribution ratio DRC (S16). Thereafter, the ratiodetermination circuit 60 temporarily ends the present processingroutine.

Next, with reference to FIG. 8 , a processing routine executed by thebrake control circuit 61 to control the operation of the brake device 15will be described. This processing routine is executed everypredetermined control cycle during vehicle braking.

In this processing routine, the brake control circuit 61 acquires thetarget vehicle body deceleration DVSTr, which is the target value of thevehicle body deceleration DVS (S21). In the vehicle braking accompanyingthe driver's braking operation (manual braking), the target vehicle bodydeceleration DVSTr increases as the braking operation amount X, which isthe operation amount of the brake operation member 16, increases. On theother hand, in the automatic braking during the automatic traveling ofthe vehicle, the target vehicle body deceleration DVSTr is determined byan application for automatic traveling. Subsequently, the brake controlcircuit 61 calculates the total braking force BPttl, which is the targetvalue for the total of the braking forces for the wheels FL, FR, RL, andRR, based on the target vehicle body deceleration DVSTr (S22).Specifically, the brake control circuit 61 calculates the total brakingforce BPttl so that the total braking force BPttl increases as thetarget vehicle body deceleration DVSTr increases.

Then, the brake control circuit 61 determines whether the ideal frontand rear braking force distribution ratio DRI calculated by the idealratio calculation circuit 59 is selected as the control front and rearbraking force distribution ratio DRC (S23). When the ideal front andrear braking force distribution ratio DRI is selected, determination ismade that the control has already shifted from the stability control tothe stable control. On the other hand, when the ideal front and rearbraking force distribution ratio DRI is not selected, determination isnot made that the control has shifted to the stable control.

Therefore, when the ideal front and rear braking force distributionratio DRI is selected (S23: YES), the brake control circuit 61 proceedsthe process to step S26 described later. On the other hand, when theideal front and rear braking force distribution ratio DRI is notselected (S23: NO), the brake control circuit 61 determines whether theslip value SLPr of the rear wheels RL, RR calculated by the slipcalculation circuit 54 is larger than a specified slip value SLPTh2(S24). The specified slip value SLPTh2 is set to a value larger than thedetermination slip value SLPTh. When the slip value SLPr has increasedto around the specified slip value SLPTh2 under a situation where thestability control is being performed, it can be assumed that the targetfront and rear braking force distribution ratio DRTr after modificationis selected as the control front and rear braking force distributionratio DRC. Then, even if the front-wheel braking force BPf and therear-wheel braking force BPr are adjusted based on the target front andrear braking force distribution ratio DRTr after modification, the brakecontrol circuit 61 determines that the suppression in the reduction ofthe stability of the vehicle behavior is not sufficient. Therefore, whenthe slip value SLPr of the rear wheels RL, RR is larger than thespecified slip value SLPTh2 (S24: YES), the brake control circuit 61performs a rear-wheel braking holding control for holding the rear-wheelbraking force BPr rather than a control using the control front and rearbraking force distribution ratio DRC (S25). In the rear-wheel brakingholding control, the brake control circuit 61 controls the brake device15 so that the front-wheel braking force BPf increases withoutincreasing the rear-wheel braking force BPr even if the total brakingforce BPttl calculated in step S22 is increased. Thereafter, the brakecontrol circuit 61 temporarily ends the present processing routine.

On the other hand, when the slip value SLPr is less than or equal to thespecified slip value SLPTh2 (S24: NO), the brake control circuit 61proceeds the process to the next step S26.

In step S26, the brake control circuit 61 executes the braking controlbased on the control front and rear braking force distribution ratioDRC. That is, when the target front and rear braking force distributionratio DRTr is selected as the control front and rear braking forcedistribution ratio DRC, the brake control circuit 61 executes thestability control. Furthermore, when the ideal front and rear brakingforce distribution ratio DRI is selected as the control front and rearbraking force distribution ratio DRC, the brake control circuit 61executes the stable control. Thereafter, the brake control circuit 61temporarily ends the present processing routine.

Next, the operation and effect of the present embodiment will bedescribed with reference to FIGS. 9 to 12 .

First, referring to FIGS. 9 and 10 , the operation and effect when theideal distribution characteristic learnt by the ideal distributioncharacteristic learning circuit 55, that is, the ideal distributioncharacteristic grasped by the brake control device 50 is not deviatedfrom the actual ideal distribution characteristic will be described.

When vehicle braking is started by the start of driver's brakingoperation or the like while the vehicle is traveling, the brake controldevice 50 starts the stability control. Then, when the brake device 15is operated by the stability control, the front-wheel braking force BPfand the rear-wheel braking force BPr are adjusted so that the actualfront and rear braking force distribution ratio DR matches the targetfront and rear braking force distribution ratio DRTr beforemodification.

Then, in the vehicle, the anti-dive force FAD corresponding to thefront-wheel braking force BPf is generated, and the anti-lift force FALcorresponding to the rear-wheel braking force BPr is generated.Furthermore, a pitching moment PM corresponding to the sum of thefront-wheel braking force BPf and the rear-wheel braking force BPr isgenerated in the vehicle. In this case, the force against the pitchingmoment PM caused by the sum of the anti-dive force FAD and the anti-liftforce FAL and the pitching moment PM act on the vehicle, and as aresult, the pitch angle PA of the vehicle approaches the target pitchangle PATr.

In the present embodiment, the front-wheel braking force BPf and therear-wheel braking force BPr are adjusted based on the target front andrear braking force distribution ratio DRTr in this way from the start ofvehicle braking. Therefore, compared with a case where the adjustment ofthe front-wheel braking force BPf and the rear-wheel braking force BPrbased on the target front and rear braking force distribution ratio DRTris started after the pitch angle PA of the vehicle is deviated from thetarget pitch angle PATr, the fluctuations in the pitch angle PA of thevehicle during braking may be prevented.

In FIG. 9 , the solid line is a characteristic line LR representing thecharacteristic of the relationship between the actual front and rearbraking force distribution ratio DR and the vehicle body decelerationDVS. Furthermore, in FIG. 9 , the broken line is the target brakingforce distribution ratio line LTr, and the one-dot chain line is theideal braking force distribution ratio line LI. Moreover, in FIG. 9 , atwo-dot chain line is a characteristic line LN representing thecharacteristic of the relationship between the distribution ratio of thefront and rear braking forces and the vehicle body deceleration DVS whenno control for distributing the front and rear braking forces isperformed. In the example shown in FIG. 9 , the rear-wheel braking forceBPr increases compared with a case where no control for distributing thefront and rear braking forces is performed by adjusting the front-wheelbraking force BPf and the rear-wheel braking force BPr based on thetarget front and rear braking force distribution ratio DRTr. Therefore,the pitch angle PA of the vehicle can be set to a value closer to thenose lift direction, as compared with a case where no control fordistributing the front and rear braking forces is performed.

The ratio calculation circuit 57 calculates the target front and rearbraking force distribution ratio DRTr so that the pitch angle of thevehicle estimated using the anti-dive force FAD and the anti-lift forceFAL generated in the vehicle during braking becomes the target pitchangle PATr. Therefore, the controllability of the pitch angle PA of thevehicle during braking can be enhanced by adjusting the front-wheelbraking force BPf and the rear-wheel braking force BPr based on thetarget front and rear braking force distribution ratio DRTr.

As shown in FIGS. 10A, 10B, and 10C, when the target vehicle bodydeceleration DVSTr increases, the total braking force BPttl iscalculated so that the vehicle body deceleration DVS of the vehicleincreases following the target vehicle body deceleration DVSTr, and thistotal braking force BPttl is distributed to each wheel FL, FR, RL, RR.At this time, in the present embodiment, when the vehicle bodydeceleration DVS of the vehicle is less than the switching decelerationDVSTh, the actual front and rear braking force distribution ratio DR isheld at the target front and rear braking force distribution ratio DRTreven if the target vehicle body deceleration DVSTr is increased. As aresult, even if the vehicle body deceleration DVS of the vehicleincreases, the pitch angle PA of the vehicle can be prevented fromdeviating from the target pitch angle PATr.

Then, the vehicle body deceleration DVS increases following the increaseof the target vehicle body deceleration DVSTr, and the vehicle bodydeceleration DVS reaches the switching deceleration DVSTh at timing t11.In the example shown in FIG. 10 , the target vehicle body decelerationDVSTr continues to increase even after timing t11. Then, in the presentembodiment, the control is switched from the stability control to thestable control, and therefore, as shown in FIG. 9 , the front-wheelbraking force BPf and the rear-wheel braking force BPr are adjusted sothat the actual front and rear braking force distribution ratio DRbecomes the ideal front and rear braking force distribution ratio DRI.Under a situation where the vehicle body deceleration DVS is larger thanthe switching deceleration DVSTh, the ideal front and rear braking forcedistribution ratio DRI is smaller than the target front and rear brakingforce distribution ratio DRTr. Therefore, as shown in FIGS. 10A, 10B,and 10C, after timing t11, the increasing speed of the rear-wheelbraking force BPr when the target vehicle body deceleration DVSTrincreases becomes small. That is, the increase of the rear-wheel brakingforce BPr is limited. Thus, even after timing t11, the lock tendency ofthe rear wheels RL, RR can be suppressed from becoming larger than thelock tendency of the front wheels FL, FR, and consequently, thereduction of the stability of the vehicle behavior can be suppressed ascompared with a case where the front-wheel braking force BPf and therear-wheel braking force BPr are adjusted based on the target front andrear braking force distribution ratio DRTr.

It should be noted that the increasing speed of the front-wheel brakingforce BPf increases as the increase of the rear-wheel braking force BPris limited in this way. Therefore, even if the control is switched fromthe stability control to the stable control, the vehicle bodydeceleration DVS of the vehicle can be suppressed from deviating fromthe target vehicle body deceleration DVSTr.

In the present embodiment, the ideal front and rear braking forcedistribution ratio DRI is updated according to the vehicle bodydeceleration DVS. That is, the ideal front and rear braking forcedistribution ratio DRI is updated so that the rear-wheel braking forceBPr is less likely to increase as the vehicle body deceleration DVSincreases. In the present embodiment, when the stable control isperformed, the front-wheel braking force BPf and the rear-wheel brakingforce BPr are adjusted so that the actual front and rear braking forcedistribution ratio DR follows the ideal front and rear braking forcedistribution ratio DRI that is sequentially updated as shown in FIG. 9 .Therefore, under a situation where the vehicle body deceleration DVS islarger than or equal to the switching deceleration DVSTh, the deviationbetween the pitch angle PA of the vehicle and the target pitch anglePATr after the stability control is terminated, as compared with a casewhere the front-wheel braking force BPf is increased but the rear-wheelbraking force BPr is not increased when the target vehicle bodydeceleration DVSTr is increased.

Here, a method of switching the control from the stability control tothe stable control based on the lock tendency of the wheels is alsoconceivable. When calculating the value representing the lock tendencyof the wheels, the vehicle body speed VS calculated based on the wheelspeed VW, the vehicle body deceleration DVS calculated based on thisvehicle body speed VS, and the like are used. During braking, thevehicle body speed VS tends to be lower than the actual vehicle bodyspeed. Therefore, the calculated value of the value representing thelock tendency of the wheels tends to be smaller than the actual valuerepresenting the lock tendency of the wheels. As a result, it isdetermined that the lock tendency of the rear wheels RL, RR is not solarge even though the lock tendency of the rear wheels RL, RR isbecoming large, and the switching from the stability control to thestable control may be delayed.

In this regard, in the present embodiment, the control switching timingis determined using the vehicle body deceleration DVS of when thestability control is being performed. As a result, when the idealdistribution characteristic grasped by the brake control device 50 isnot deviated from the actual ideal distribution characteristic, an eventin which the control switching timing is delayed and the stability ofthe vehicle behavior reduces is less likely to occur, as compared with acase where the control switching timing is determined based on the locktendency of the rear wheels RL, RR.

Next, with reference to FIG. 11 and FIG. 12 , the operation and effectwhen the ideal distribution characteristic grasped by the brake controldevice 50 is deviated from the actual ideal distribution characteristicwill be described. Note that, in FIG. 11 , the ideal braking forcedistribution ratio line LI representing the ideal distributioncharacteristic grasped by the brake control device 50 is represented bya one-dot chain line, and the ideal braking force distribution ratioline LIr representing the actual ideal distribution characteristic isrepresented by a two-dot chain line.

When vehicle braking is started by the start of driver's brakingoperation or the like while the vehicle is traveling, the brake controldevice 50 starts the stability control. Then, when the brake device 15is operated by the stability control, the front-wheel braking force BPfand the rear-wheel braking force BPr are adjusted so that the actualfront and rear braking force distribution ratio DR matches the targetfront and rear braking force distribution ratio DRTr beforemodification.

Since the ideal distribution characteristic grasped by the brake controldevice 50 is deviated from the actual ideal distribution characteristic,as shown in FIGS. 12A, 12B, and 12C, the slip value SLPr of the rearwheels RL, RR becomes larger than or equal to the determination slipvalue SLPTh even though the slip value SLPf of the front wheels FL, FRis less than the determination slip value SLPTh at timing t21 when thevehicle body deceleration DVS of the vehicle is less than the switchingdeceleration DVSTh. Then, the target front and rear braking forcedistribution ratio DRTr is modified so that the increase of therear-wheel braking force BPr is limited. The broken line in FIG. 11 is atarget braking force distribution ratio line LTr representing thecharacteristic of the relationship between the target front and rearbraking force distribution ratio DRTr before modification and thevehicle body deceleration DVS.

Therefore, after timing t21, the front-wheel braking force BPf and therear-wheel braking force BPr are adjusted based on the target front andrear braking force distribution ratio DRTr after modification. Then, asshown in FIGS. 12A, 12B, and 12C, the slip value SLPr of the rear wheelsRL and RR is less likely to increase as the increase amount of therear-wheel braking force BPr is reduced. That is, as shown by the brokenline in FIG. 12B, the changing speed of the wheel deceleration DVW ofthe rear wheels RL, RR becomes small. As a result, reduction in thestability of the vehicle behavior while performing the stability controlmay be prevented.

After timing t21, the increase amount of the front-wheel braking forceBPf increases as the increase in the rear-wheel braking force BPr islimited. Therefore, the vehicle body deceleration DVS of the vehicle canbe made to follow the target vehicle body deceleration DVSTr in the samemanner as before timing t21.

In the example shown in FIG. 12 , even if the front-wheel braking forceBPf and the rear-wheel braking force BPr are controlled based on thetarget front and rear braking force distribution ratio DRTr aftermodification, the slip value SLPr of the rear wheels RL, RR increases asthe vehicle body deceleration DVS increases. Then, at timing t22, theslip value SLPr of the rear wheels RL, RR exceeds the specified slipvalue SLPTh2. As a result, after timing t22, the execution of thestability control is terminated, and the rear-wheel braking holdingcontrol for holding the rear-wheel braking force BPr is started. Then,after timing t22, the rear-wheel braking force BPr is not increased evenif the target vehicle body deceleration DVSTr is increased. Therefore,the slip value SLPr of the rear wheels RL and RR can be reduced.Therefore, the vehicle can be decelerated while the vehicle behavior isstabilized.

Note that after timing t22, the front-wheel braking force BPf is greatlyincreased as the rear-wheel braking force BPr is not increased when thetarget vehicle body deceleration DVSTr is increased. Therefore, thevehicle body deceleration DVS of the vehicle can be made to follow thetarget vehicle body deceleration DVSTr in the same manner as beforetiming t22.

In the present embodiment, the following effects can be furtherobtained.

-   -   (1) In the present embodiment, the ideal distribution        characteristic is learnt based on the slip value SLPr of the        rear wheels RL, RR and the slip value SLPf of the front wheels        FL, FR during braking. Therefore, the ratio determination        circuit 60 can set the switching deceleration DVSTh to an        appropriate value. As a result, the shift of the control from        the stability control to the stable control may be performed at        an appropriate timing.    -   (2) In the present embodiment, the target pitch angle PATr is        changed between the time of automatic braking of the vehicle and        the time of vehicle braking accompanying the driver's braking        operation. The driver is likely to feel the deceleration feeling        of the vehicle by generating a pitching moment PM in the nose        dive direction in the vehicle when the vehicle is decelerating.        Therefore, during braking accompanying the braking operation,        the target pitch angle PATr is set to a value in the nose dive        direction. Therefore, the deceleration feeling of the vehicle        can be given to the driver through changes in the orientation of        the vehicle. On the other hand, at the time of automatic braking        of the vehicle, the target pitch angle PATr is set to suppress        the change in the pitch angle PA accompanying the vehicle        deceleration. Therefore, the comfortability of the vehicle        occupant during the automatic traveling of the vehicle may be        improved.

The present embodiment can be modified and implemented as follows. Thepresent embodiment and the following modified examples can beimplemented in combination with each other within a technicallyconsistent scope.

The target pitch angle reference value PATrB at the time of automaticbraking of the vehicle may be the same as the target pitch anglereference value PATrB during braking accompanying the driver's brakingoperation. Furthermore, the target pitch angle reference value PATrB atthe time of automatic braking of the vehicle may be a value in the nosedive direction of the target pitch angle reference value PATrB duringbraking accompanying the driver's braking operation.

In the embodiment described above, the pitch angle modification amountΔPA used when modifying the target pitch angle reference value PATrB tocalculate the target pitch angle PATr is calculated based on the idealfront and rear braking force distribution ratio DRI at the initial stageof braking and the gradient θ of the road surface on which the vehicletravels. However, if the pitch angle modification amount ΔPA iscalculated based on the ideal front and rear braking force distributionratio DRI at the initial stage of braking, the gradient θ of the roadsurface may not be used for calculating the pitch angle modificationamount ΔPA.

Furthermore, if the pitch angle modification amount ΔPA is calculatedbased on the gradient θ of the road surface, the ideal front and rearbraking force distribution ratio DRI at the initial stage of braking maynot be used.

When calculating the target pitch angle PATr, the ideal front and rearbraking force distribution ratio DRI at the initial stage of braking andthe gradient θ of the road surface on which the vehicle travels may notbe used.

The target pitch angle PATr may be fixed at a predetermined value set inadvance.

As the determination slip value SLPTh, a plurality of values havingdifferent sizes may be prepared. For example, of the plurality ofdetermination slip values SLPTh, the smallest value is set as the firstdetermination slip value, and the value larger than the firstdetermination slip value is set as the second determination slip value.Then, when the slip value SLPr of the rear wheels RL, RR becomes largerthan or equal to the first determination slip value even though the slipvalue SLPf of the front wheels FL, FR is less than the firstdetermination slip value under the situation where the stability controlis performed, the modification amount ΔDR may be made equal to the firstspecified value smaller than “0”. Then, the sum of the target front andrear braking force distribution ratio DRTr calculated by the ratiocalculation circuit 57 and such a modification amount ΔDR may be set asthe target front and rear braking force distribution ratio DRTr aftermodification.

Then, when the slip value SLPr of the rear wheels RL, RR becomes largerthan or equal to the second determination slip value even though theslip value SLPf of the front wheels FL, FR is less than the seconddetermination slip value under the situation where the front-wheelbraking force BPf and the rear-wheel braking force BPr are adjustedbased on the target front and rear braking force distribution ratio DRTrafter modification, the modification amount ΔDR may be made equal to thesecond specified value smaller than the first specified value. Then, thesum of the target front and rear braking force distribution ratio DRTrcalculated by the ratio calculation circuit 57 and such a modificationamount ΔDR may be set as the target front and rear braking forcedistribution ratio DRTr after modification.

That is, during the execution of the stability control, the target frontand rear braking force distribution ratio DRTr may be modified inmultiple stages in accordance with the increase in the slip value SLProf the rear wheels RL, RR.

If set to a value of less than or equal to “0”, the modification amountΔDR used in modifying the target front and rear braking forcedistribution ratio DRTr may gradually decreases as the slip value SLProf the rear wheels RL, RR increases. For example, the absolute value ofthe modification amount ΔDR may be made larger as the slip value SLPrincreases by calculating the modification amount ΔDR based on thedeviation between the slip value SLPf of the front wheels FL, FR and theslip value SLPr of the rear wheels RL, RR.

In the embodiment described above, the slip value, which is thedifference between the wheel deceleration DVW and the vehicle bodydeceleration DVS, is used as the value that represents the lock tendencyof the wheels. However, other values other than the slip value may beadopted as the value representing the lock tendency of the wheels aslong as it represents the lock tendency of the wheels. For example, theslip amount, which is the difference obtained by subtracting the wheelspeed VW from the vehicle body speed VS, may be adopted as the valuerepresenting the lock tendency of the wheels.

In the embodiment described above, if the switching deceleration DVSThis set to a value corresponding to the vehicle body deceleration whenthe ideal braking force distribution ratio line LI and the targetbraking force distribution ratio line LTr intersect, the switchingdeceleration DVSTh may be a value different from the vehicle bodydeceleration when the ideal braking force distribution ratio line LI andthe target braking force distribution ratio line LTr intersect. Forexample, a value obtained by subtracting an offset value from thevehicle body deceleration when the ideal braking force distributionratio line LI and the target braking force distribution ratio line LTrintersect may be set as the switching deceleration DVSTh.

The switching deceleration DVSTh may be fixed at a predetermineddeceleration.

The stable control may not cause the front and rear braking forcedistribution ratio DR to follow the ideal front and rear braking forcedistribution ratio DRI that is periodically updated by the ideal ratiocalculation circuit 59 if the front and rear braking force distributionratio DR can be made smaller than the target front and rear brakingforce distribution ratio DRTr before modification. For example, thestable control may be control that increases the front-wheel brakingforce BPf but does not increase the rear-wheel braking force BPr even ifthe target vehicle body deceleration DVSTr is increased.

When both the braking force and the driving force are not applied to thevehicle and the vehicle is traveling by inertia, the ground contact loadof the wheels can be regarded as equivalent to the sprung load and thespring load, and thus in the embodiment described above, the pre-brakingpitch angle PAb is calculated based on the ground contact load FWf ofthe front wheels FL and FR and the ground contact load FWr of the rearwheels RL and RR. However, this is not the sole case, and the sprungload and the spring load may be calculated, and the pre-braking pitchangle PAb may be calculated based on the calculated sprung load and thespring load under a situation where the vehicle is traveling by inertia.

When calculating the target front and rear braking force distributionratio DRTr, the pre-braking pitch angle PAb may not be taken intoconsideration. The pitching during braking can be suppressed similar tothe embodiment described above by performing the stability control usingthe target front and rear braking force distribution ratio DRTr duringbraking.

The ideal distribution characteristic learning circuit 55 may learn theideal distribution characteristic through a method different from thelearning method described in the above embodiment. For example, theideal distribution characteristic learning circuit 55 can calculate thevehicle weight based on the motion state of the vehicle while thevehicle is traveling, and can estimate the ideal braking forcedistribution based on the calculated vehicle weight. In this learningmethod, the vehicle weight is calculated from the relationship betweenthe driving force at the time of vehicle acceleration and theacceleration of the vehicle, the ground contact load of the front wheelsFL, FR and the ground contact load of the rear wheels RL, RR areestimated based on this vehicle weight, and the ideal distributioncharacteristics are estimated based on the estimated results of theground contact load of each wheel FL, FR, RL, RR. For example, when thecalculated vehicle weight corresponds to the vehicle weight when theoccupant is on board, it can be presumed that there is an occupant inthe rear portion of the vehicle body, that is, the occupant is seated inthe rear seat, and the ground contact load of the rear wheels RL, RR isincreased. In this case, the ideal distribution characteristic learningcircuit 55 determines that there is a possibility that the front wheelsFL, FR may lock before the rear wheels RL, RR, and for example, storesthe characteristic that can be represented by the ideal braking forcedistribution ratio line LI indicated by a solid line in FIG. 5 as idealdistribution characteristic.

The ideal distribution characteristic learning circuit 55 may learn theideal distribution characteristic through the learning method describedin the above embodiment during braking, and the learning method based onthe vehicle weight at the time of non-braking of the vehicle. In thiscase, since the ideal distribution characteristic can be learnt not onlyduring braking but also at the time of vehicle traveling, when the idealdistribution characteristic grasped by the brake control device 50deviates from the actual ideal distribution characteristic, thedeviation can be promptly resolved.

The relationship between the WC pressure Pwc and the braking forceapplied to the wheels changes due to wear of the rotating body 13 andthe friction material 14 that form the braking mechanism 11. Therefore,the brake control device 50 may have a function of modifying therelationship between the WC pressure Pwc and the braking force appliedto the wheels. In this case, the target front and rear braking forcedistribution ratio DRTr based on the target pitch angle PATr may becalculated with reference to the relationship between the WC pressurePwc and the braking force applied to the wheels modified by the relevantfunction. Thus, the controllability of the pitch angle PA of the vehiclewhen performing the stability control can be further enhanced.

Here, an example of a method of modifying the relationship between theWC pressure Pwc and the braking force applied to the wheels will bedescribed. The modification is performed when the pitching moment PM ishardly generated in the vehicle even when the vehicle is braked. Thatis, when the specified deceleration is set as the target vehicle bodydeceleration DVSTr, the front-wheel braking force BPf and the rear-wheelbraking force BPr are adjusted so that the front and rear braking forcedistribution ratio DR becomes the first distribution ratio. The vehiclebody deceleration DVS of the vehicle at this time is defined as thefirst vehicle body deceleration.

Furthermore, when the prescribed deceleration is set as the targetvehicle body deceleration DVSTr on another occasion, the front-wheelbraking force BPf and the rear-wheel braking force BPr are adjusted sothat the front and rear braking force distribution ratio DR becomes thesecond distribution ratio larger than the first distribution ratio. Thevehicle body deceleration DVS of the vehicle at this time is defined asthe second vehicle body deceleration.

Then, for example, when the second vehicle body deceleration issubstantially equal to the prescribed deceleration, and the firstvehicle body deceleration is smaller than the second vehicle bodydeceleration, determination can be made in the braking mechanism 11 forthe front wheels FL, FR that the braking force is less likely toincrease with respect to an increase in the WC pressure Pwc. Therefore,based on such a determination result, the relationship between the WCpressure Pwc and the front-wheel braking force BPf in the brakingmechanism 11 for the front wheels FL and FR is modified.

Moreover, for example, when the first vehicle body deceleration issubstantially equal to the prescribed deceleration, and the secondvehicle body deceleration is smaller than the first vehicle bodydeceleration, determination can be made in the braking mechanism 11 forthe rear wheels RL, RR that the braking force is less likely to increasewith respect to an increase in the WC pressure Pwc. Therefore, based onsuch a determination result, the relationship between the WC pressurePwc and the front-wheel braking force BPf in the braking mechanism 11for the rear wheels RL and RR is modified.

1. A brake control device for a vehicle applied to a brake deviceconfigured to adjust a front-wheel braking force that is a braking forceapplied to a front wheel of the vehicle and a rear-wheel braking forcethat is a braking force applied to a rear wheel of the vehicle, thebrake control device for the vehicle comprising: a ratio calculationcircuit that calculates a target front and rear braking forcedistribution ratio that is a target value of a front and rear brakingforce distribution ratio based on a target pitch angle, the front andrear braking force distribution ratio is a ratio of the rear-wheelbraking force with respect to the front-wheel braking force, and thetarget pitch angle is a target value of a pitch angle of the vehicleduring braking; and, a brake control circuit that performs a stabilitycontrol by operating the brake device based on front and rear brakingforce distribution ratio during braking, wherein in the stabilitycontrol, the brake control circuit operates the brake device to hold thefront and rear braking force distribution ratio at the target front andrear braking force distribution ratio even when a vehicle bodydeceleration of the vehicle changes.
 2. The brake control device for thevehicle according to claim 1, wherein an anti-dive force that displacesa front portion of the vehicle upward and an anti-lift force thatdisplaces a rear portion of the vehicle downward are generated in thevehicle during braking, the anti-dive force is a force whose absolutevalue increases as the front-wheel braking force increases, and theanti-lift force is a force that increases as the rear-wheel brakingforce increases, and the ratio calculation circuit calculates the targetfront and rear braking force distribution ratio so that a pitch angle ofthe vehicle estimated from the anti-dive force and the anti-lift forcegenerated in the vehicle during braking becomes the target pitch angle.3. The brake control device for the vehicle according to claim 1,wherein the brake control circuit performs a stable control that lowersthe front and rear braking force distribution ratio less than the targetfront and rear braking force distribution ratio by operating the brakedevice if the vehicle body deceleration of the vehicle is larger than aswitching deceleration during braking, and an ideal front and rearbraking force distribution ratio that is the front and rear brakingforce distribution ratio at which the front wheel and the rear wheel aresimultaneously locked, in a graph in which one of a vertical axis and ahorizontal axis indicates the front-wheel braking force and the otherindicates the rear-wheel braking force, the switching deceleration isset to a value corresponding to a vehicle body deceleration of thevehicle when a line representing a relationship between the target frontand rear braking force distribution ratio and the vehicle bodydeceleration of the vehicle, and a line representing a relationshipbetween the ideal front and rear braking force distribution ratio andthe vehicle body deceleration of the vehicle intersect.
 4. The brakecontrol device for the vehicle according to claim 3, further comprising:a learning circuit that learns an ideal distribution characteristic thatrepresents the relationship between the ideal front and rear brakingforce distribution ratio and the vehicle body deceleration of thevehicle during braking; wherein the learning circuit learns the idealdistribution characteristic based on a lock tendency of the rear wheelsand a lock tendency of the front wheels during vehicle braking.
 5. Thebrake control device for the vehicle according to claim 3, furthercomprising: a learning circuit that learns an ideal distributioncharacteristic that represents the relationship between the ideal frontand rear braking force distribution ratio and the vehicle bodydeceleration of the vehicle during traveling; wherein the learningcircuit learns the ideal distribution characteristic based on a vehicleweight obtained based on a motion state of the vehicle during traveling.6. The brake control device for the vehicle according to claim 3,further comprising, a slip value that is a value representing a degreeof slip of the wheel, a ratio modification circuit that modifies thetarget front and rear braking force distribution ratio so thatdistribution of the braking force to the rear wheel decreases when theslip value of the front wheel is less than a determination slip valueand the slip value of the rear wheel is larger than the determinationslip value under a situation where the stability control is performed bythe brake control circuit.
 7. The brake control device for the vehicleaccording to claim 1, further comprising a target pitch angle settingcircuit that sets the target pitch angle to forward during automaticbraking less than the target pitch angle to forward during manualbraking.
 8. The brake control device for the vehicle according to claim2, wherein the brake control circuit performs a stable control thatlowers the front and rear braking force distribution ratio less than thetarget front and rear braking force distribution ratio by operating thebrake device if the vehicle body deceleration of the vehicle is largerthan a switching deceleration during braking, and an ideal front andrear braking force distribution ratio that is the front and rear brakingforce distribution ratio at which the front wheel and the rear wheel aresimultaneously locked, in a graph in which one of a vertical axis and ahorizontal axis indicates the front-wheel braking force and the otherindicates the rear-wheel braking force, the switching deceleration isset to a value corresponding to a vehicle body deceleration of thevehicle when a line representing a relationship between the target frontand rear braking force distribution ratio and the vehicle bodydeceleration of the vehicle, and a line representing a relationshipbetween the ideal front and rear braking force distribution ratio andthe vehicle body deceleration of the vehicle intersect.
 9. The brakecontrol device for the vehicle according to claim 8, further comprising:a learning circuit that learns an ideal distribution characteristic thatrepresents the relationship between the ideal front and rear brakingforce distribution ratio and the vehicle body deceleration of thevehicle during braking; wherein the learning circuit learns the idealdistribution characteristic based on a lock tendency of the rear wheelsand a lock tendency of the front wheels during vehicle braking.
 10. Thebrake control device for the vehicle according to claim 8, furthercomprising: a learning circuit that learns an ideal distributioncharacteristic that represents the relationship between the ideal frontand rear braking force distribution ratio and the vehicle bodydeceleration of the vehicle during traveling; wherein the learningcircuit learns the ideal distribution characteristic based on a vehicleweight obtained based on a motion state of the vehicle during traveling.